The present invention relates to a high brightness AlGaInP light-emitting diode which comprises an upper cladding layer including three layers with different electron concentrations, and a metal oxide window layer.
As a light-emitting device with a band from green, yellow to reddish orange, an aluminum.gallium.indium phosphide (AlGaInP) light-emitting diode (LED) with a pn junction-type double-hetero (DH) junction structure is known (Appl. Phys. Lett., 61(15)(1992), pp. 1775-1777). In particular, (Alxcex1Ga1xe2x88x92xcex1)0.5In0.5P (0xe2x89xa6xcex1xe2x89xa61) with an indium composition ratio of about 0.5 has an advantage of being capable of achieving excellent lattice matching with a gallium arsenide (GaAs) single crystal (Appl. Phys. Lett., 57(27)(1990), pp. 2937-2939). Therefore, it is used to construct a cladding layer which constitutes a light-emitting portion with a DH junction structure, or a light-emitting layer (active layer)(Appl. Phys. Lett., 58(10)(1991), pp. 1010-1012).
The LED in which the cladding layer on the side from which light is emitted is constructed of an n-type layer is referred to as n-side-up type LED, while the LED in which the cladding layer on the side from which light is emitted is constructed of a p-type layer is referred to as p-side-up type LED. In any of the above layered structures, the cladding layers which hold the light-emitting layer therebetween extend over a large area of the light-emitting layer, and are usually composed of (Alxcex1Ga1xe2x88x92xcex1)0.5In0.5P (0xe2x89xa6xcex1xe2x89xa61) doped with an n-type or p-type impurity with high concentration in order to cause a device operating current to flow (Japanese Laid-Open Patent Application 2-168690). Furthermore, the electron concentration or hole concentration inside the cladding layers is generally made approximately constant (Japanese Laid-Open Patent Application 2-260682).
In a conventional high brightness AlGaInP LED, a window layer for efficiently picking out light emission from the light-emitting portion is provided above the upper cladding layer (SPIE, Vol. 3002 (1997), pp. 110-118). The window layer is required to be constructed of a semiconductor material which is transparent to the light emitted, and has a large forbidden band gap. Conventionally, an example constructed of aluminum.gallium arsenide crystal (AlcGa1xe2x88x92cAs: 0xe2x89xa6Cxe2x89xa61) is known (the above-mentioned Appl. Phys. Lett., 58(1991)) and an example constructed of gallium phosphide (J. Electron. Mater., 20 (1991), pp. 1125-1130).
In addition to the III-V group compound semi-conductor materials, for example, U.S. Pat. No. 5,481,122 discloses the use of a metal oxide window layer constructed of indium-tin oxide (abbreviated to ITO). Furthermore, there is disclosed means for providing a transparent oxide layer composed of a film made of indium oxide, tin oxide, zinc oxide, or magnesium oxide (Japanese Laid-Open Patent Application 11-17220). Some metal oxide has a room temperature forbidden band gap as high as above 3 eV, and is better in picking out light emission compared to the III-V group compound semiconductors conventionally used as the construction materials for the window layer.
A problem that is caused when a transparent window layer made of a metal oxide with an n-type conductivity, such as ITO, is provided on the upper cladding layer composed of an n-type (Alxcex1Ga1xe2x88x92xcex1)0.5In0.5P (0xe2x89xa6xcex1xe2x89xa61) by junction, is that a metal oxide layer with a low ohmic contact resistance cannot be formed in a stable manner. For this reason, conventionally, a contact layer with high n-type carrier concentration (=electron concentration) which exceeds about 1xc3x971019 cmxe2x88x923 is usually provided on the n-type cladding layer (the above-mentioned Japanese Laid-Open Patent Application 11-17220). The contact layer is constructed of gallium arsenic phosphide (GaAP), gaium phosphide (GaP), gallium indium phosphide (GaInP), or gallium arsenide (GaAs) (the above-mentioned Japanese Laid-Open Patent Application 11-17220) and has a problem that the layered structure thereof is complex.
The present invention has been made in view of the above-mentioned problems in the prior art, and provides an n-side-up type AlGaInP light-emitting diode provided with a window layer made of a metal oxide, wherein the light-emitting diode is a high brightness LED with a simple layered structure.
The inventors of the present invention have studied diligently to solve the above-mentioned problems, and finally arrived at the present invention. The present invention is directed to the light-emitting diodes described below:
(1) A light-emitting diode comprising a p-type GaAs single crystal substrate on which a p-type lower cladding layer, a light-emitting layer, and an n-type upper cladding layer are provided, each layer being represented by (AlXGa1xe2x88x92X)YIn1xe2x88x92YP (0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61)(including the case where each layer has different mixed crystal ratios X, Y), and a window layer composed of a metal oxide, characterized in that the n-type upper cladding layer includes three layers, each layer having a different electron concentration (including the case where each of the three layers has different mixed crystal ratios X, Y, the three layers being referred to as a first n-type layer (with an electron concentration of n1 and a layer thickness of d1), a second n-type layer (with an electron concentration of n2 and a layer thickness of d2), and a third n-type layer (with an electron concentration of n3 and a layer thickness of d3) counted in the order from the layer on the side near the light-emitting layer), with the electron concentrations of the three layers being in the relationship of n3 greater than n1 greater than n2.
(2) The light-emitting diode as described in (1) above, wherein the mixed crystal ratio (1xe2x88x92Y) of In in each of the p-type lower cladding layer, the light-emitting layer, and the n-type upper cladding layer is 0.5.
(3) The light-emitting diode as described in (1) or (2) above, wherein the window layer is in contact with the third n-type layer.
(4) The light-emitting diode as described in any of (1) to (3) above, wherein the layer thicknesses of the first to third n-type layers are in the relationship of d1 greater than d3xe2x89xa7d2.
(5) The light-emitting diode as described in any of (1) to (4) above, wherein the electron concentration of the second n-type layer is 1xc3x971016 cmxe2x88x923xe2x89xa6n2xe2x89xa65xc3x971017 cmxe2x88x923.
(6) The light-emitting diode as described in any of (1) to (5) above, wherein the layer thickness of the second n-type layer is 20 nmxe2x89xa6d2xe2x89xa6200 nm.
(7) The light-emitting diode as described in any of (1) to (6) above, wherein a dopant for the first and second n-type layers is Si, and a dopant for the third n-type layer is Se or Te.
(8) The light-emitting diode as described in any of (1) to (7) above, wherein the window layer has an n-type conductivity.
(9) The light-emitting diode as described in any of (1) to (8) above, wherein the window layer is constructed of at least one material selected from the group consisting of indium-tin oxide, indium oxide, tin oxide, zinc oxide, and magnesium oxide.
(10) The light-emitting diode as described in (9) above, wherein the zinc oxide of which the window layer is constructed comprises one of aluminum, gallium, or indium.
(11) The light-emitting diode as described in any of (1) to (10) above, wherein the p-type GaAs single crystal substrate has an electron concentration in the range of 5xc3x971018 to 5xc3x971019 cmxe2x88x923.